The issue with using a 9-1/4 in an A body is shortening the rears cost. By the time your done paying people to shorten it, you can have a 8-3/4 in there.
An 8-1/4 is reasonably strong. It'll handle 400HP. No problem.
I'm not sure what it cost to have a housing narrowed around you but here its only $125 however, I'm not sure what new axles would cost.
I do know around here everyone think the 8-3/4 is gold and most appear to asking in the $2000+ range for a complete assembly in good condition but half the time they have the small bolt pattern axles so I would still have to buy new axles
